1. Field of the Invention
The present invention relates to hybrid drives, in particular axle hybrids.
2. Description of Related Art
Modern hybrid vehicles such as so-called axle hybrids are equipped with hybrid drives, which may include an electric motor and a combustion engine. Axle hybrids are characterized by the fact that one of the vehicle axles is driven by one drive source, for example a combustion engine, and the other vehicle axle is driven by another drive source, for example an electric motor. In this context, the electric motor may operate as a generator, in the event of a braking operation for example, in order to recuperate the braking energy and to store it in a vehicle battery. Furthermore, the electric motor may be used to drive a switchable drive axle in the event of an increased slip of the wheels, as disclosed in the printed document published German patent document DE 3542059 C1.
To drive the vehicle, a propulsive torque is normally generated as a function of a gas or accelerator pedal position, which in an axle hybrid is always composed of a sum of the torques applied on the respective vehicle axles. Thus, at an accelerator pedal position of 0%, for example, a slight deceleration sets in, which is connected with a negative propulsive torque, a so-called overrun state of −20 Nm for example, a propulsive torque of 0 Nm sets in at an accelerator pedal position of approximately 10%, and a positive propulsive torque, a so-called traction state, sets in at an accelerator pedal position of above 10%.
When the sign of the propulsive torque changes, which is connected for example with a change of the propulsive torque from an overrun to a traction state, or in an acceleration reversal of the vehicle, in which a sign of the accelerating force is changed, a load reversal occurs, which may result in a torque zero crossing. The torque zero crossing may cause oscillations in the affected drive train, which may impact the operating behavior of the vehicle negatively or may be disagreeably noticeable to the driver. One possibility of reducing the load reversal oscillations is to slow down the zero crossing artificially, which has the consequence, however, that the vehicle following an accelerator pedal position at less than 10% is only able to accelerate again after a delay.